The present invention generally relates to microwave dispersive lines and in particular to wideband, low loss dispersive lines.
Wideband, low loss dispersive lines operated at microwave or higher frequencies are increasingly utilized in analog signal processing systems. Such systems include post detection receivers of high performance radars, spread-spectrum communications and spectral analyzers. A dispersive line provides delay at microwave frequencies. An input signal including signals of different frequencies is launched as a forward-propagating wave on the line. At specified points along the line a portion of the signal energy is reflected in a backward-propagating wave along the line. Signals of different frequencies are reflected back along different points on the line, thereby providing different delay periods for different frequencies.
Existing dispersive delay lines typically utilize either surface-acoustic-wave (SAW) technology, or high temperature superconductive (HTS) backward coupler technology. Both technologies, however, have several important disadvantages when used in high frequency wideband signal processing applications. The SAW technology can only be utilized for signal frequencies of up to 1 GHz which is substantially below microwave frequencies of 2 GHz or higher. The physical dimensions of SAW circuits are impractical for frequencies of 2 GHz or higher. Further, the tolerances of SAW circuits are inadequate for the demands of high performance signal processing systems.
Although HTS backward coupler circuits can operate at microwave or higher frequencies, for a wideband, long dispersive delay line, multiple directional couplers and interconnecting uncoupled transmission lines are required. Each coupler must be developed before it is cascaded in a coupler circuit unit, requiring substantial development costs. Further, to fit the circuits into HTS wafers, the multiple coupled and uncoupled transmission lines must be bent at the same time into two meander lines. As such, the circuits are of substantial size and do not make efficient usage of the HTS wafers since only a small portion of the surfaces of the wafers are utilized.
There is, therefore, a need for a wideband, low loss, long dispersive delay line which can operate at microwave frequencies or higher without requiring large circuit dimensions. There is also a need for such a delay line to be cost effective to fabricate and make efficient use of wafer space.